Phosphoinositide 3-kinase (PI3K), class IA, delta isoform, catalytic domain; The PI3K catalytic domain family is part of a larger superfamily that includes the catalytic domains of other kinases such as the typical serine/threonine/tyrosine protein kinases (PKs), aminoglycoside phosphotransferase, choline kinase, and RIO kinases. PI3Ks catalyze the transfer of the gamma-phosphoryl group from ATP to the 3-hydroxyl of the inositol ring of D-myo-phosphatidylinositol (PtdIns) or its derivatives. PI3Ks can be divided into three main classes (I, II, and III), defined by their substrate specificity, regulation, and domain structure. Class I PI3Ks are the only enzymes capable of converting PtdIns(4,5)P2 to the critical second messenger PtdIns(3,4,5)P3. Class I enzymes are heterodimers and exist in multiple isoforms consisting of one catalytic subunit (out of four isoforms) and one of several regulatory subunits. They are further classified into class IA (alpha, beta and delta) and IB (gamma). Class IA enzymes contain an N-terminal p85 binding domain, a Ras binding domain, a lipid binding C2 domain, a PI3K homology domain of unknown function, and a C-terminal ATP-binding cataytic domain. They associate with a regulatory subunit of the p85 family and are activated by tyrosine kinase receptors. PI3Kdelta is mainly expressed in immune cells and plays an important role in cellular and humoral immunity. It plays a major role in antigen receptor signaling in B-cells, T-cells, and mast cells. It regulates the differentiation of peripheral helper T-cells and controls the development and function of regulatory T-cells.